Shallow groundwaters in weathered volcanic, granitic and sedimentary rocks in relation to dryland salinity in southern New South Wales

Abstract

Soils affected by secondary salinization were studied in six areas on the Southern Tablelands of New South Wales. All the salt-affected areas are underlain by, or occur in close proximity to, deeply weathered volcanic, granitic and sedimentary rocks which commonly contain stores of soluble salts, dominantly sodium chloride. The chemical composition of shallow groundwaters in the areas was monitored by piezometers for periods of up to two years. Water levels in the piezometers responded rapidly to rainfall, but the ionic composition of the waters generally remained fairly uniform. All waters are dominated by sodium chloride; those with the highest contents occurred in volcanic and granitic rocks, followed by Ordovician sediments and the lowest contents were in Silurian sediments. The chlorine contents in samples of weathered rocks follow a similar sequence. Electron microprobe analyses indicate that the chlorine-bearing minerals in the unaltered rocks are principally biotite, hornblende and potassium, sodium and calcium feldspars. No salt-affected soils were found in areas underlain by unweathered rocks.